How do I explain about my difficult child?

I have 3 kids and the middle one is very challenging. I am receiving support from a psychologist through child health however there are stressful times. For example when we visited a park with a number of other mums from my older child's school. My middle son hit and pinched another child very badly. I was mortified and humiliated. We left the park. I feel like it is difficult to make Mum friends and connect with other families when my son's behavior can be so unpredictable. How do I explain to others his volatile nature with putting him in a box and risking him labelled as the 'bad' kid. I feel guilty for making this about me and my connection to others. I know we have issues to sort and I am trying my very best to sort them. However I feel alienated by others due mostly to his behavior. My husband says don't worry if your 'friends' don't understand they are not worth having. But it's not that simple- these are lovely families and have children who are friends of my older child.